1. Field of the Invention
This invention relates to an optical wavelength converter element for converting a fundamental wave into a second harmonic wave and, more particularly, a method and apparatus for creating domain reversals in a predetermined pattern on a ferroelectric substance having a non-linear optical effect in order to fabricate an optical wavelength converter element possessing periodic domain reversals.
2. Description of the Prior Art
A proposal has already been made by Bleombergen et al. in Physics Review vol. 127, No. 6 in 1918 (1962), in which the wavelength of a fundamental wave is converted into a second harmonic wave using an optical wavelength converter element with regions (domains) where the spontaneous polarization of a ferroelectric substance possessing a nonlinear optical effect is periodically inverted.
In this method, the fundamental wave can be phase matched with the second harmonic wave by setting the period .LAMBDA. of the domain reversals to be an integral multiple of a coherence length .LAMBDA.c which is given by EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1)
where .beta.(2.omega.) designates the propagation constant of the second harmonic wave, and 2.beta.(.omega.) represents the propagation constant of the fundamental wave. When wavelength conversion is effected using a bulk crystal made of a nonlinear optical material, a wavelength to be phase-matched is limited to a specific wavelength inherent to the crystal. However, in accordance with the above described method, phase matching can be efficiently carried out by selecting a period .LAMBDA. which satisfies the condition (1) for an arbitrary wavelength.
One example of the known methods for creating such periodic domain reversals is disclosed in U.S. Pat. No. 5,415,743 (filed by the same applicant as the present invention), in which an electrode is formed in a predetermined pattern on one surface of a unipolarized ferroelectric substance having a non-linear optical effect, and the ferroelectric substance is subjected to corona electrical charging by the electrode and a corona wire disposed above another surface that is opposite to the electrode-side surface, so that an electric field is applied to the substrate. Thus, localized domain reversals are created at areas where the electrode faces the ferroelectric substance.
The method for creating ferroelectric domain reversals utilizing the corona electrical charging is superior in productivity when compared with a method in which electron beams are irradiated in a predetermined pattern onto a ferroelectric substance. However, it is widely admitted that the method utilizing the corona electrical charging is inferior in evenness in the thickness-wise direction of the domain reversals and reproducibility.
In order to improve the evenness in the thickness-wise direction of the domain reversals and the reproducibility, as described in the foregoing U.S. Pat. No. 5,415,743 it has already been put forward that an electric field is applied to a ferroelectric substance positioned in a vacuum. However, this conventional method is only applied to a case where an electric field is directly applied to a ferroelectric substance via electrodes which are formed on two opposite surfaces of the ferroelectric substance, but is not applied to a case where an electric field is applied to a ferroelectric substance utilizing corona electrical charging. In other words, it is impossible for the ferroelectric substance to be subjected to corona electrical charging in a vacuum, and hence it has been deemed that it is impossible to apply the application of an electric field in a vacuum to the method for creating domain reversals utilizing corona electrical charging in view of the underlying principle.